Explosion &amp; fire suppression system for catalytic reactors

ABSTRACT

A system for detecting ignition of combustibles in catalytic reactors and appurtenant equipment and for injecting a gaseous suspension of inert finely-divided solids to the combustion site to suppress combustion.

OR 397709059 SR United States Patent 1 1191 T e Graham Nov. 6, 1973 [54] EXPLOSION & FIRE SUPPRESSlON SYSTEM 888,073 5/1908 Elliott et a1. 169 2 ux FOR CATALYTlC REACTORS 3,482,637 12/1969 Mitchell et ai.. 169/28 X 3,465,827 9/1969 Levy et al. 169/2 R [75] Inventor: Jam s J- Gr h m, W yl n Mass- 3,614,987 10 1971 Bonne et a1 169 2 A 3,135,330 6/1964 Hanson et a1... 169/9 X [73] Assgnee- T Bafiger Company 3,033,291 5/1962 Wieslandcr"... 169 1 A Cambrdge Mass 3,289,686 12 1966 Tyler, .Ir. 169/42 x 22 Filed; 8 1971 3,568,775 3/1971 Greenberg 169/20 3,269,352 8/1966 Van Winkle 137/557 X 21 Appl. NO; 113,297

Primary ExaminerM. Henson Wood, Jr. 52 US. Cl 169/1 A 169/5 302/53 Assis'am 51 Int. Cl A626 35/00 Pandiscio [58] Field of Search 169/1 A, 2 R, 5,

169/9, 16; 137/557;302/53; ZOO/61.25; 220/9 [57] ABSTRACT A system for detecting ignition of combustibles in cata- [56] References Cit d lytic reactors and appurtenant equipment and for in- UNITED STATES PATENTS jecting a gaseous suspension of inert finely-divided sol- 3 545 547 12/1970 lrgon 169/2 R x ids to the combustion site to suppress combustion. 2,692Z649 10/1954 McCrear 169/9 X 18 Claims, 4 Drawing Figures PAIENIEDNuv 6 I975 3. 770.059

sum 2 BF 3 INVENTOR.

JAMES J. GRAHAM J palm Maia ATTORNEYS PMENTED NOV 6 I975 sum 3 CF 3 TO I43 "FIG. 4

INVENTOR.

JAMES J. GRAHAM Az/[b (if Q L ATTORNEYS EXPLOSION & FIRE SUPPRESSION SYSTEM FOR CATALYTIC REACTORS This invention relates to prevention of explosions in chemical reaction apparatus and more particularly to an improvement in the art of preventing and extinguishing fires and explosions in catalytic reactors and associated chemical process equipment.

Many chemical reaction systems operate within the flammability limits of one or more of the reactants and are susceptible to ignition and explosion. Often reaction systems which normally operate outside of the flammability limits are also susceptible to ignition because of accumulation of combustible material on reactor surfaces or surfaces of appurtenant equipment such as condensers. It is common practice to inject liquid or gaseous flame suppressants into a reaction system to extinguish a flame front. Common types of suppressants for chemical reactor systems include water, steam, nitrogen, and carbon dioxide. Other suppressants are fire extinguishing foams and inorganic salts such as sodium bicarbonate, sodium chloride, and potassium chloride. These and other suppressants suffer from one or more disadvantages such as being limited in use to particular chemical reaction systems, or having limited effectiveness, and/or producing other undesirable consequences. Thus certain commercial suppressants cannot be used in catalytic reactors because they liberate noxious vapors or tend to form reactive mixtures with one or more of the reactants or deposit solids that are harmful to the catalyst, or contaminate thereaction products so that they are unuseable. Water and steam have thewidest use as suppressants but are not suitable for injection into certain reaction systems. Moreover, since the benefit derived from water is due primarily to its heat capacity, a large volume is required to be injected to extinguish or prevent combustion in a reactor or associated equipment.

l-leretofore, work has been done by the US. Bureau of Mines and others on the use of inert dust as flame suppressants for combustible atmospheres in mines. Particulate suppressants have also been used to retard or extinguish open tires and to extinguish highly exothermic reactions in nuclear reactors. Examples of prior efforts along these lines are presented by Bureau of Mines Report of Investigations 6,543 entitled Preventing Ignition of Dust Dispersions by Inerting and US. Pat. Nos. 1229064, 29691 16, 3207672, and 3333896.

I have determined that inert dusts may be used advantageously to prevent or extinguish ignition in chemical reactor equipment and accordingly the primary object of this invention is to provide new and improved systems and methods for suppressing flames and explosions in chemical reaction process equipment and particularly in catalytic reactors and appurtenant appara- Still another object is to provide a new and improved method and apparatus for preventing and extinguishing tires and explosions in catalytic reactors and associated chemical process equipment characterized by relative simplicity and low cost and no basic changes required in reactor and associated process equipment design.

The foregoing and other objects obvious to persons skilled in the art are achieved by providing detector means for detecting an incipient flame or explosion at a selected site in a catalytic reactor system and means responsive to the detector means for injecting a selected solid suppressant in particle form into said system at the region of said selected site to suppress the incipient flame or explosion. Provision may be made for purging combustibles (or combustion products) and suppressants from the reaction system. The suppressant is non-combustible and is compatible with the catalyst used in the reactor systems. In a preferred embodiment of the invention the suppressant consists of catalyst fines. However, the suppressant also may be flne particles of the catalyst carrier or base'material or one or more constituents of the catalyst. The supply of suppressant is maintained in a pressure chamber and is blown into the reactor by an inert carrier gas. Flow of carrier gas may be continued after the particulate suppressant has been injected so as to dilute the combustible mixture and thereby further reduce the likelihood of reignition.

Other features and advantages of the invention are described or rendered obvious by the following detailed description which is to be considered together with the accompanying drawings wherein:

FIG. 1 illustrates a fixed bed catalytic reactor embodying a flame suppression system according to the present invention;

FIG. 2 illustrates a fluid bed catalytic reactor system embodying the present invention;

FIG. 3 illustrates a fluid bed catalytic reactor incorporating another embodiment of the invention; and

FIG. 4 illustrates a liquid phase reactor embodying the invention. I

Referring now to FIG. 1, there is shown one type of fixed catalyst bed reactor commonly used in the chemical process industry. In this case the reactor comprises a reactor vessel 2 provided with an inlet pipe 4 for admitting reactants to the reaction system. Mounted within the vessel 2 are two partitions in the form of plates 8 and 10 which subdivide the vessel into upper and lower plenum chambers 12 and 14 and a third chamber 16. Supported in chamber 16 between plates 8 and 10 is a plurality of tubes 18. Although three tubes l8-are shown, it is to be appreciated that the number of tubes is a matter of choice and is not critical to the invention. The bottom ends of the tubes 18 are secured to and protrude through the plate 10. The upper ends of the tubes are mounted in and protrude above the plate 8. The plates 8 and 10 are unperforated and act as a fluid barrier. A coolant fluid (or heating fluid if reaction is endothermic) enters the vessel 2 through the pipe 6, flows down the outside of the tubes removing the heat of reaction (or adding heat to the reaction if the latter is endothermic) and leaves through pipe 6A. Each of the tubes 18 is filled with a suitable catalyst 20 in particle form that is supported on a screen or perforated plate 22 secured to the bottom end of the tube. As is obvious to persons skilled in the art, the reactants enter the tubes via plenum 14, contact the catalyst, and

then leave the tubes through their upper ends which open into the upper plenum chamber 12. The reaction effluent is removed from the vessel via an outlet pipe 24.

As is well known to persons skilled in the art, explosions or fires often occur in the inlet or outlet plenum chambers. Fires may be initiated by a spark or by pyrophors accumulated in the equipment or as a result of the system being operated close to or within the flammability limits of one or more of the reactants. The reactor of FIG. 1 has been adapted with a system according to the present invention for sensing a flame in the inlet plenum chamber 14 and for extinguishing a flame and suppressing combustion by introduction of a predetermined amount of inert (i.e., non-combustible) dust. The flame suppression system of FIG. 1 comprises an ultra violet light detector 26 which is mounted to the vessel 2 so as to monitor the inlet plenum chamber. Detector 26 is adapted to produce an electrical signal when ultra-violet light is detected in plenum chamber 14. Ultra violet light detectors generally are provided with a viewing aperture covered by a lens. The detector may be mounted directly in an opening in the vessel. Preferably, however, it is mounted with its viewing lens attached to one end of a length of pipe 27 which is mounted in the wall of the vessel. The pipe 26 acts as a viewing element and also functions as a heat radiator to protect the detector from being damaged by heat from the reactor.

Also connected to the inlet plenum chamber is a pipe 28 which leads to a pressure vessel 30 which contains a supply of an inert dust represented schematically at 32 introduced via a suitable manhole (not shown). The vessel 30 may include an interior screen or perforated plate 34 located near the vessels bottom end for supporting the supply of dust 34. The bottom end of vessel 30 is fitted with an inlet line 36 leading to a supply of an inert gas under pressure (not shown). Line 28 is provided with a control valve 38 which is operated by a solenoid 40. As an optional measure, line 36 may also have a solenoid controlled valve 38A. The solenoids 40 are operated by current from a current source (not shown) applied through a control circuit comprising a relay 42 which is actuated by the output of detector 26 amplified by an amplifier 44. I l

The above-described system may be operated according to two different modes. In the preferred mode the valve 38 is normally closed and the valve 38A in line 36 is omitted or replaced by a normally open manually operable shut-off valve so that the vessel 30 is pressurized with inert gas. When a flame or spark is sensed by the detector 26, the detector produces an output signal which after amplification actuates relay 42 so that current is applied to energize the solenoid 40 of valve 38, whereupon the latter is opened. When this occurs, the gas pressure in vessel 30 causes the dust to be blown into the inlet plenum chamber 14 to prevent further ignition of combustibles and suppress any existing flame in the plenum chamber. So long as valve 38 remains open, the inert gas will continue to flow into the inlet plenum chamber to dilute any combustible mixture therein and thereby prevent reignition.

The relay may be adapted to keep solenoid 40 of valve 38 energized until the relay is manually reset or may be connected to amplifier 44 by automatic means including a timer (not shown) which actuates the relay in response to the signal output of detector 26 and resets the relay to shut off valve 38 after the latter has been open a predetermined period of time sufficient to effect flame suppression as hereinabove described.

In the second mode of operation, lines 28 and 36 each include solenoid valves 38 which are both normally closed. In this case the supply of dust is supported on screen 34 which acts as a gas distributing grid for the inert gas and the pressure in the vessel is substantially atmospheric. When a flame or spark is sensed by detector 26, the latter produces an output electrical signal that actuates relay 42 to pass current to the solenoids 40 of both valves 38 and 38A, whereby both valves open so that the inert gas can pass through the vessel 30 to the inlet plenum chamber 14. The dust is swept out of the vessel as a suspension in the gas and is blown into the reactor to suppress combustion in the same manner as in the first mode of operation. Reclosing of valves 38 and 38A may be effected manually or automatically as in the first mode of operation. It is to be noted that the first mode of operation is preferred over the second mode since pre-pressurizing vessel 30 cuts down the amount of time required to inject a given amount of dust into the reactor.

The system of FIG. 1 may be modified by providing reactant feed line 4 with an additional control valve (not shown) connected so as to be closed automatically when ,a spark or flame is detected by detector 26. In any event, it is to be noted that a predetermined amount of inert dust may be stored in vessel 30, so that the amount of dust introduced to the reactor system is limited and known. In the case of a fixed bed reactor, the catalyst dust will not enter the tubes 18 if the relative particle size of the dust and catalyst is of the same order. If the dust particles are small enough for them to enter the catalyst tubes 18, they can be removed by blowing a gas through the catalyst tubes in reverse fashion.

It is to be recognized also that the vessel 30 may be connected by an auxiliary pipe line to the upper plenum chamber 12 so that dust may be selectively injected into that section if a flame condition occurs therein. In the latter case, a detector like detector 26 is mounted in the upper end of the vessel 2 to monitor the upper plenum section and an auxiliary pipeline is provided with a valve adapted to be opened like valve 38 when a flame is detected in the upper plenum chamber. A similar arrangement may be used to monitor and suppress a flame in the reactor chamber 13.

It is to be noted that the flame suppression system of FIG. 1 may be used even if the reactor is operated in the reverse fashion, i.e., so that reactants are introduced at the top and the reaction effluent removed from the bottom. Similarly, the same flame suppression system may be adapted to a fixed catalyst reactor wherein the tubes 18 are horizontal or where the catalyst is disposed in a single bed.

Turning now to FIGS. 2 and 3, the invention also is adaptable to suppressing combustion in fluid bed catalytic reactors and appurtenant equipment. In FIG. 2 there is shown a reactor comprising a vessel 50 provided with a distributing grid 52 on which is supported a bed of suitable catalyst 53. The vessel is provided with one or more inlet pipes 54 for introducing reactants to the catalyst bed and also an inlet pipe 56 through which a suitable fluidizing gas is introduced at a rate such as to fluidize the bed of catalyst according to well known techniques. The broken line 58 represents the upper level of the dense phase catalyst bed. The space 60 above the bed is the catalyst disengaging zone which is occupied by a dilute suspension of catalyst in the reaction effluent. Depending upon the particular reaction to be carried out in the reactor, the fluidizing gas may be an inert gas or it may be a reactant used in the reaction process. According to standard practice, the reactor also includes conventional means for separating catalyst fines from the effluent vapors. By way of example, catalyst fines may be separated by means of one or more stationary filters or preferably by a cyclone separator 64. Separated fines are returned to the dense phase catalyst bed via a dip-leg 66 and the dust-free effluent is removed via a line 68 leading to a condenser 70. After heat exchange in the condenser 70, the effluent passes through to other processing or collecting equipment (not shown) via line 72. Heating or cooling fluid (as required) passes into and out of the condenser via lines 74 and 76.

It is common in fluidized catalyst reaction systems of the type shown in FIG. 2 for fire and explosions to develop in the dust-free effluent as it travels from the reactor through succeeding equipmentin which pyrophors tend to collect. In the embodiment of FIG. 2, provision has been made according to the present invention to detect a fire in the condenser 70 and to suppress such fire by injection of dust. In this case, a line 78 is provided between the disengaging zone 60 of the vessel and a point in the line 68 upstream of but close to the condenser. Line 78 includes a valve 80 operated by a solenoid 82 under the control of a timer operated switch unit 84. The unit 84 is connected to respond to the output of an amplifier 86 which amplifies a signal generated by an ultra violet light detector 90. The latter is mounted so as to monitor the line 68 'at the entrance to the condenser 70. Alternatively, the detector may be mounted directly to the condenser so as to monitor the chamber of condenser 70 through which flows the effluent from the reactor.

Valve 80 is normally closed and is opened by energization of solenoid 82 from a current source (not shown) which is switched on by unit 84 in response to the output of detector 90. It is to be noted that a pressure drop exists between the disengaging zone 60 and the line 68 at the point of connection of line 78. Accordingly, when the valve 80 is opened as a result of detection of a flame by detector 90, the vapor and the entrained catalyst fines in the disengaging zone will flow through the line 78 into the condenser 70. The entrained catalyst fines injected into the condenser act to suppress the flame. The unit 84 shuts off the valve 80 after a predetermined time interval. If at the time the valve 80 is reclosed the fire still exists in the condenser,

the detector 90 will sense the continuing flame and will act through amplifier 86 and unit 84 to reopen the valve to repeat the cycle. Once the fire has been extinguished, the detector 90 will no longer produce a signal output to start the timer of unit 84 and hence, the valve 80 will close and remain closed after the timer has run out.

It is to be noted that the system of FIG. 2 may be modified by providing shut-off valves in the lines 54 and 56 which are closed automatically whenever a flame is detected and which reopen with valve 80. alternatively, lines 54 and/or 56-may include throttling valves which reduce the .rate of flow of reactants and fluidizing gas when valve is opened and restore the normal rate of flow when valve 80 is reclosed.

FIG. 3 shows the same fluid bed reactor 50 of FIG. 2. However, in this case, provision is made for detecting the existence of ignition or a flame in the upper end of the reactor and for injecting an inert dust into the reactor itself. Accordingly, an ultra violet light detector 92 is mounted to monitor the disengaging zone 60. Alternatively, the detector 92 may be connected to line 68 just where it leaves the cyclone 64. At this point the catalyst fines have been removed from the effluent gas by the cyclone and fires are known to occur. This is termed after burning in the petroleum industry.

Still referring to FIG. 3, a tank 94 is provided for storing a supply of inert dust 95. The tank 94 is connected by a line 96 to the upper end of the reactor. Line 96 is provided with a valve 98 controlled by a solenoid 100. The bottom end of the vessel 94 has an inlet pipe 102 for admission of inert gas from a supply of gas under pressure (not shown). Line 102 includes a rate controlling valve 104 which is controlled by an actuator 106. The latter is under the control of a pressure regulator 108 which is connected to monitor the pressure in tank 94. The tank 94 is fitted with a bleeder line 110 which is capped by a filter member 1 12 which allows the inert gas to escape while preventing release of dust. Regulator 108 causes the actuator 106 to vary the setting. of valve 104 in accordance with the pressure in tank 94 so that inert gas will flow into the tank 94 at a rate such as to maintain the dust in a fluidized condition.

The effluent outlet line 68 is provided with shut-off valve 112 and also a by-pass line 114. Another shut-off valve 116 is mounted in line 114. Valves 112 and 116 are operated by solenoids 118 and 120 which, together with solenoid 100, are controlled by a relay 122 connected to respond to the output of an amplifier 124 connected to the detector 92.

The system of FIG. 3 functions as follows: During normal operation valve 112 is open, valves 98 and 116 are closed, and valve 104 is opened enough to permit sufficient gas to vessel 94 to maintain the dust in a fluidized condition. When a flame is detected, the detectorv 92 produces a signal which is amplified enough to actuate relay 122 so that the solenoids 100,118, and 120 are energized by current from a suitable current source (not shown), whereupon valves 98, 112, and 116 reverse positions (i.e. valve 112 closes and valves 116 and 98 open). As soon as valve 98 is opened, gas and dust will flow into the reactor via line 96. At the same time the pressure in tank 94 will drop with the result that the regulator 108 will cause the actuator 106 to open valve 104 in an attempt to restore the pressure in tank 94 to its original level. The increased flow of gas through the tank 94 will complete the expulsion of dust from the tank and into the disengaging zone of the reactor. Inert gas will continue to flow into the reactor through tank 94 after all of the dust has been injected until valve 98 is reclosed. Reclosing of valve 98 may be accomplished by manual or automatic resetting of relay 122 as described above in connection with the systems of FIGS. 1 and 2. During the time that valve 116 is open, the products of combustion are by-passed out of the system via line 114. Thus the combustibles and combustion gases are automatically purged from the system so that little or no contamination of the product results.

The invention is applicable to still other types of chemical reactors, including reactors in which the catalytic medium is not in the form of a fixed or fluidized bed of solid particles. Thus, with reference to FIG. 4, the invention is applicable to a liquid phase reactor comprising a vessel 130 that is provided with suitable means for stirring the liquid phase 132. By way of example, the stirring means may comprise a rotatable shaft 134 having an impellor 136 at its bottom end. The shaft 134 is connected to and driven by a motor drive unit 140 attached to the upper end of the vessel. Reactant feed is introduced to the vessel via an inlet line 141 provided with a valve 142 controlled by a solenoid 143, and the gaseous reaction effluent is removed from the vessel via a line 144. This type of reactor vessel may be used, for example, to hydrogenate unsaturated organic compounds (unsaturated alcohols, acids, ketones, etc.) with the liquid phase solution 132 comprising a Raney nickel catalyst. In accordance with this invention the reactor is provided with an ultraviolet detector 146 mounted so as to monitor its interior above the liquid phase. The detector provides an electrical output which is applied to a control unit 148. The latter may comprise a relay and amplifier as employed in the flame suppression systems of FIGS. I- 3 or other components designed to provide the same control functions. Control unit 148 is connected to control energization of solenoid 143 and also energization of a solenoid 150 of a solenoid control valve 152 that is mounted in a line 154 connecting the upper end of the reactor chamber with the upper end of a pressure vessel 156 in which is disposed a suitable supply of inert dust. Vessel 156 is constructed similarly to the vessel 94 of FIG. 3 and also includes the same arrangement of pressure regulator 108, valve actuator 106, valve 104 connected in the gas inlet line 102, and bleeder line 110 fitted with a filter cap 112. Additional quantities of dust may be admitted to vessel 156 via a supply line 166 leading from a supply hopper 170. A valve 172 in line 160 isolates the supply hopper 170 from the pressure vessel 156 when .the latter contains an ample quantity of dust and permits additional dust to be admitted to the vessel on command after the initial supply has been injected into the reactor. The flame suppression system of FIG. 4 operates in a similar manner to the system of FIG. 3. When a flame is detected in the reactor the control circuit 148 causes the solenoid 150 to open the valve 152 and solenoid 143 to close valve 142. Assuming that valve 104 is normally partly open so that the dust within the vessel is maintained in a fluidized state, when valve 152 opens the supply of dust is blown from vessel 156 into the reactor to suppress combustion. The reduced pressure in the vessel 156 causes the actuator 106 to fully open valve 104 so that the inert gas continues to flow into the reactor at a rate sufficient to assure that all of the dust in the vessel 156 is injected into the reactor and also to dilute combustibles in the reactor. The unit 148 may be designed so as to automatically reopen valve 142 and reclose valve 152 when a flame is no longer sensed by detector 146 or after the valve 152 has been open for a predetermined period of time. As an optional measure, the valve 172 may be provided with an appropriate control which permits it to automatically feed an additional supply of dust to the vessel 156 when valve 152 is reclosed, with provision being made to terminate flow of inert gas into vessel 156 before valve 172 is opened and to restore flow of gas into the vessel after a new supply of dust has been delivered from hopper 170.

It is to be noted that the dust which is used for suppression of combustion in the foregoing systems must be non-combustible and must not be consumable by reaction with the reactant supplied to the reactor, the reaction products, or the catalyst within the reactor. Preferably the dust consists of fines of the catalyst medium itself where the catalyst medium is a solid in particle form. Alternatively the dust may consist of the active catalyst or the catalyst carrier or base material where the catalyst medium comprises a catalyst on an inert carrier. By way of example, in a case where the catalyst used in. the reactor is a metal oxide on an inert carrier such as silica gel or porous alumina, the inert dust may consist of the metal oxide per se or the carrier or the metal oxide on the carrier. Introduction of dust consisting of the catalyst medium or the catalyst per se or the catalyst carrier offers the advantage that it is compatible with the catalyst (and hence will not destroy its activity) and also with the reactants and reaction products.

It is preferred that the dust have a particle size in the range of O to microns and preferably 0 to 40 microns. Larger size dust particles cannot be injected as fast, may cause clogging or unduly restrict the future flow of reactants through the reaction apparatus, and are less effective in suppressing combustion.

The amount of dust required to be injected into the system depends upon the size of the reactor (or appurtenant equipment where such equipment is monitored according to the present invention) but should be sufficient to suppress the combustion reaction. The duration of inert gas flow should be long enough to purge combustibles from the ignition or combustion site and preferably long enough to purge combustibles out of the reaction system. An excessive amount of dust extends the time required to purge it from the system and may restrict future flow of reactants and reaction effluent. In any event, it has been found that the amount of dust required to be injected to suppress a flame front is substantially less than one would expect to be the case if the suppression benefit were due simply to the heat capacity of the dust (the heat capacity of the inert gas is negligible). Furthermore the required amount of dust can be injected to the combustion site within 100-200 milliseconds after the combustion is detected.

The fact that only a small amount of dust is required to prevent ignition of combustible mixtures is illustrated by the following example. Two vessels each with a volume of 1 cubic foot were filled with like combustible mixtures of 2 vol phthalic anhydride, 12 vol. oxygen and 86 vol. nitrogen at atmospheric pressure and temperature. Additionally 0.3 lbs. of fines of a noncombustible catalyst medium having an average particle size of 20 microns was injected into and maintained dispersed in one of the vessels. The catalyst medium consisted of vanadium oxide supported on a porous silica carrier. Both vessels were heated to a temperature of 700F which was sufiicient to insure complete vaporization of the phthalic anhydride. The catalyst medium to vapor ratio by weight was 3.7 to 1.0. A spark was produced in the vessel having no catalyst and the mixture therein immediately ignited. A peak pressure of 100 psig. was recorded indicating that a combustion explosion had occurred in the vessel. Sparks were repeatedly produced in the other vessel but no ignition occurred and no pressure rise was evident, indicating that the catalyst fines had suppressed ignition.

It is to be noted that the required inert gas pressure will vary depending upon the pressure within that portion of the chemical process equipment into which the dust is to be injected (the inert gas pressure must be greater) and also the average size and specific gravity of the dust particles. The rate of flow of the gas must be sufficient to carry all of the supply of dust into the reactor within a brief period of time and also to accomplish dilution of the combustible vapor mixture in the reactor. Optimum pressure and rates of flow of the inert gas must be determined for each reactor system.

It is to be noted that the several embodiments described above show different features which maybe combined in one chemical reactor system. Thus, for example, it is contemplated that the reactor system of FIG. 1 may include (1) a shutoff or throttling valve in the reactant input line 4 which is shut off or adjusted to lower the feed rate of reactants automatically when a fire is detected in reactor 2 or within appurtenant equipment such as a condenser installed in the effluent delivery line 24, (2) a by-pass line arrangement as disclosed in FIG. 3 for by-passing combustibles and combustion products, (3) an arrangement as disclosed in FIGS. 3 and 4 whereby the supply of dust is maintained in a fluidized condition in the vessel 30 until the time when the dust is to be delivered to a combustion site. It also is to be understood that a number of flame detectors may be used to monitor different sites in the chemical reactor system and that such detectors may be connected to the same or different dust supply storage vessels.

Various types of flame detectors may be used in practicing the invention. Ultraviolet light detectors are preferred since they are capable of accurately distinguishing sparks and flames. By way of example, Geiger- Mueller detector tubes may be used as ultraviolet light detectors. Other types of ultraviolet light detectors are commercially available, notably ultraviolet detectors manufactured by Fenwal, Inc. of Ashland, Massachusetts and McGraw-Edison Types 6l2 and 613 U/V fire protection systems. The control circuits may be simple amplifier-relay arrangements as herein described or other more sophisticated control systems may be used. The control circuit arrangement also may gauges, and hot filament ionization gauges, and resistive or capacitive pressure transducers (see Perrys I Chemical Engineers Handbook, Fourth Edition, pages be adapted to activate an alarm in response to the outdetector-control circuit arrangement for operating the several valves (e.g., the combination of detector 146 and control unit 148 in FIG. 4) may be of the type illustrated in V. K. Zworykin and E. G. Ramberg, Photoelectricity," page 431, John Wiley and Sons, Inc., 1949, which includes a relay adapted to activate an alarm; the same circuit may be used to activate the other relays for closing and opening valves in accordance with the present invention.

Still other types of detectors may be used. For example, it is known that in certain spontaneous combustion reactions, the combustion is preceded by a sharp rise in vapor pressure or temperature. Accordingly, a pressure or temperature detector may be used to monitor likely sites of combustion and to provide a signal output to activate the control system when the monitored pressure or temperature undergoes a sharp rise. Suitable thermal detectors are pyrometers, bolometers, thermistors, and thermocouples; and suitable pressure 22-4 to 22-20, McGraw-Hill, 1963).

It is to be noted further that the control valves, e.g., valve 98, may be adapted for operation by pneumatic or hydraulic means rather than a solenoid. Typical pneumatic and hydraulic actuators, regulators, and controllers are disclosed in Perrys Chemical Engineers Handbook, pages 22-76 to 22-87 inclusive.

The choice of gas used to inject the flame suppressing dust depends upon the particular chemical reaction system to which the invention is applied. Preferably the gas used is inert with respect to the catalyst and process reactants and reaction process. Among the gases that may be used'toinject dust into a number of chemical reaction systems are nitrogen and argon. Other gases that may be used are obvious to persons skilled in the art. It also is contemplated that a gas may be used which is not inert but which reacts with the reactants in a manner that suppresses the primary reaction in the reactor and thus prevents formation of reaction products that are combustible or support combustion.

It is to be noted also that the dust need not consist of all or part of the catalyst medium used in the reactor but could be some other inert (i.e., non-combustible) particulate material which is compatible with the chemical process system. A dust material which is compatible with the chemical process system is one that has no deleterious effect on the system and, more specifically, does not react with the systems reactants, reaction products, catalyst medium (if the dust is not a component compound of the catalyst) or equipment. However, such materials are used only where the catalyst medium is not a particulate solid or in situations such as a liquid phase reactor employing a Raney nickel catalyst where the catalyst is pyrophoric. Use of a dust consisting of fines of the catalyst per se and/or the catalyst carrier offers the further advantage that in many cases wheresome catalyst fines normally appear in the reaction effluent, the dust may be left in the effluent without any deleterious affect; alternatively the dust may be recovered from the effluent and returned to the catalyst bed in the reactor. If the dust is a material other than the catalyst per se and/or the catalyst carrier, recovery of the dust from the reaction effluent may or may not be required, depending upon such factors as type and amount of dust used, ease and cost of recovering the dust, and whether or not the reaction effluent is subjected to further processing.

In addition to the advantages already mentioned, it is to be noted that the invention permits quick suppression of combustion in continuous reaction systems without need for shutting down operations. On the other hand it also is applicable to chemical process systems that operate on a batch basis. Furthermore the invention provides protection of chemical process systems operating within or close to flammable limits and permits chemical process systems which normally operate outside of flammable limits (i.e., under conditions at which combustion will not occur) to exceed flammable limits during startup and shutdown.

Modifications other than those already mentioned will 'be obvious to persons skilled in the art from the foregoing descriptions. The essential thingis that the invention is applicable to reactors and appurtenant equipment used in many different chemical processes and particularly processes involving exothermic reactions and operations close to the flammability limits of reactants and reaction products. As used herein, the term chemical process" denotes processes in which selected materials are subjected to chemical reaction (with or without consumption or use of other reactants) to yield one or more reaction products, and thus embraces but is not limited to operations such as hydrogenation, oxidation, desulfurization, dehydrogenation, alkylation, and cracking. Further by way of example, the invention may be applied to (a) the production of phthalic anhydride by vapor phase oxidation of O-xylene (in a fixed or fluid bed reactor) using a catalyst consisting of vanadium oxide supported on silica (or one of the other catalysts described in U.S. Pat. No. 2,954,385) and (b) the production of acrylonitrile by reaction of propylene, ammonia, and oxygen in contact with a fluidized solid catalyst such as bismuth phosphomolybdate on a silica or alumina carrier (or one of the other catalysts described in U.S, Pat. No. 3,197,419 or British Pat. No. 1,172,958). In (a) flame suppression is effected by injecting the catalyst fines with nitrogen gas; in (b) the catalyst is preferably injected with nitrogen gas.

What is claimed:

1. In a chemical reaction system comprising a chemical reactor vessel in which one or more selected reactions occur in the presence of a selected catalyst, means for delivering reactants to said reactor vessel and conduit means for removing reaction products from said reactor vessel, improved means for suppressing combustion of flammable materials in said system, said improved means comprising detector means for sensing combustion at a selected site within said system, and means responsive to said detector means for injecting into said system at said site a gaseous suspension of a non-combustible solid material in particle form in an amount sufficient for said non-combustible material to suppress said combustion, said catalyst and said non-comnbustible material comprising material of the same composition.

2. The combination of claim 1 wherein the catalyst consists of solid particles.

3. The combination of claim 1 wherein said noncombustible solid material is suspended in an inert gas.

4. The combination of claim 1 wherein said injecting means includes a second vessel containing a supply of said non-combustible material, means for supplying gas to said vessel to produce said gaseous suspension and conduit means for delivering said suspension from said second vessel to said site.

5. The combination of claim 1 wherein said detector means is adapted to detect ultra-violet light and further wherein said injecting means is rendered operative when ultra-violet light is sensed by said detector means.

6. The combination of claim 1 wherein said site is a region within said reactor.

7. The combination of claim 1 wherein said catalyst is maintained in a fluidized state within said reactor.

8. The combination of claim 1 wherein said detector means is a pressure-responsive transducer adapted to operate-said injecting means when the pressure in said system at said site exceeds a predetermined level.

9. The combination of claim 1 further including means for terminating delivery of reactants to said reactor vessel when said injecting means is rendered operative.

10. The combination of claim 1 further including means for purging combustion products from said system when said injecting means is rendered operative.

11. The combination of claim 1 further including means for rendering said injecting means inoperative after said injecting means has been in operation for a predetermined period of time.

12. In combination with chemical reaction apparatus including a chemical reactor containing a selected catalyst in particle form, means for introducing at least one reactant to said reactor to contact said catalyst, and means for removing unreacted reactant and reaction product materials from said reactor, the improvement comprising rneans for injecting into said system on command a quantity of non-combustible dust sufficient to suppress combustion of combustibles in said system, said catalyst and said non-combustible dust comprising material of the same composition.

13. Method of suppressing a combustion of combustible materials in a chemical process system in which a predetermined chemical reaction is effected by contacting at least one chemical reactant with a catalyst, said method comprising monitoring said system to detect ignition of combustible materials and upon detection of ignition injecting into said system a quantity of non-combustible dust sufficient to suppress combustion of said combustible materials, said catalyst and said non-combustible dust comprising material of the same composition.

14. Method according to claim 13 wherein said dust is injected as a suspension in an inert gas.

15. Method according to claim 13 wherein said catalyst is in particle form.

16. Method according to claim 14 wherein said dust consists of catalyst fines.

17. Method according to claim 13 further including purging said system of any combustion products and said dust.

18. Method of suppressing combustion of combustible materials in a chemical process system comprising a catalyst and at least one chemical reactant, where such combustion is detrimental to said process, comprising introducing to said system at a point at or upstream of the site of combustion a non-combustible dust compatible with said process in an amount sufficient to suppress said combustion, said catalyst and said non-combustible dust comprising material of the same composition. 

1. In a chemical reaction system comprising a chemical reactor vessel in which one or more selected reactions occur in the presence of a selected catalyst, means for delivering reactants to said reactor vessel and conduit means for removing reaction products from said reactor vessel, improved means for suppressing combustion of flammable materials in said system, said improved means comprising detector means for sensing combustion at a selected site within said system, and means responsive to said detector means for injecting into said system at said site a gaseous suspension of a non-combustible solid material in particle form in an amount sufficient for said non-combustible material to suppress said combustion, said catalyst and said noncomnbustible material comprising material of the same composition.
 2. The combination of claim 1 wherein the catalyst consists of solid particles.
 3. The combination of claim 1 wherein said non-combustible solid material is suspended in an inert gas.
 4. The combination of claim 1 wherein said injecting means includes a second vessel containing a supply of said non-combustible material, means for supplying gas to said vessel to produce said gaseous suspension and conduit means for delivering said suspension from said second vessel to said site.
 5. The combination of claim 1 wherein said detector means is adapted to detect ultra-violet light and further wherein said injecting means is rendered operative when ultra-violet light is sensed by said detector means.
 6. The combination of claim 1 wherein said site is a region within said reactor.
 7. The combination of claim 1 wherein said catalyst is maintained in a fluidized state within said reactor.
 8. The combination of claim 1 wherein said detector means is a pressure-responsive transducer adapted to operate said injecting means when the pressure in said system at said site exceeds a predetermined level.
 9. The combination of claim 1 furTher including means for terminating delivery of reactants to said reactor vessel when said injecting means is rendered operative.
 10. The combination of claim 1 further including means for purging combustion products from said system when said injecting means is rendered operative.
 11. The combination of claim 1 further including means for rendering said injecting means inoperative after said injecting means has been in operation for a predetermined period of time.
 12. In combination with chemical reaction apparatus including a chemical reactor containing a selected catalyst in particle form, means for introducing at least one reactant to said reactor to contact said catalyst, and means for removing unreacted reactant and reaction product materials from said reactor, the improvement comprising means for injecting into said system on command a quantity of non-combustible dust sufficient to suppress combustion of combustibles in said system, said catalyst and said non-combustible dust comprising material of the same composition.
 13. Method of suppressing a combustion of combustible materials in a chemical process system in which a predetermined chemical reaction is effected by contacting at least one chemical reactant with a catalyst, said method comprising monitoring said system to detect ignition of combustible materials and upon detection of ignition injecting into said system a quantity of non-combustible dust sufficient to suppress combustion of said combustible materials, said catalyst and said non-combustible dust comprising material of the same composition.
 14. Method according to claim 13 wherein said dust is injected as a suspension in an inert gas.
 15. Method according to claim 13 wherein said catalyst is in particle form.
 16. Method according to claim 14 wherein said dust consists of catalyst fines.
 17. Method according to claim 13 further including purging said system of any combustion products and said dust.
 18. Method of suppressing combustion of combustible materials in a chemical process system comprising a catalyst and at least one chemical reactant, where such combustion is detrimental to said process, comprising introducing to said system at a point at or upstream of the site of combustion a non-combustible dust compatible with said process in an amount sufficient to suppress said combustion, said catalyst and said non-combustible dust comprising material of the same composition. 